1. Field of the Invention
The present invention relates to a method for producing a liquid composition containing pigment particles, a method for producing pigment particles, and an ink composition for ink jet recording, containing the liquid composition produced by the producing method.
2. Description of the Related Art
Recent years, digital printing technology is remarkably progressing.
The digital printing technology typified by electrophotographic technology and ink jet technology is increasingly important as image forming technology in offices and for home use.
In the field of electrophotographic technology, attention is being attracted to an electrophotographic photosensitive member utilizing an organic photoconductive substance (hereinafter referred to as an organic photosensitive member).
The organic photosensitive member has advantages such that it has a satisfactory film forming property and can be produced by a coating process, has an extremely high productivity and can be produced at a low cost, and that the sensitivity can be arbitrarily controlled by the selection of an organic pigment to be used as a sensitizer.
Most of the organic photosensitive members are composed of at least two layers, a charge generation layer and a charge transport layer, in which carriers generated by light in the charge generation layer are injected into the charge transport layer to neutralize charges on the surface of the photosensitive member, thereby generating an electrostatic image.
Electrophotographic characteristics such as image characteristics depend significantly on the charge generation layer, and the magnitude of carrier generation, the uniformity of carrier generation and how to efficiently inject the generated carriers into the charge transport layer become key factors in the development of the organic photosensitive member.
It is already known that the characteristics of the charge generation layer are influenced, in general, by the particle size of an organic pigment serving as a charge generation substance.
A smaller particle size is considered to provide satisfactory electrophotographic characteristics, as it increases a charge generating efficiency by a surface area effect and allows a uniform and smooth charge generation layer to be formed.
On the other hand, also in the field of ink jet technology, the image forming characteristics are considered to be influenced by the particle size of an organic pigment.
As an example of the ink jet technology, there is known a method of heating an ink supplied from an ink tank, by a heater provided in a nozzle, thereby causing evaporation and forming a bubble, to discharge the ink to form an image on a recording medium.
As another example, there is known a method of causing a vibration in a piezo element, thereby discharging an ink from the nozzle.
These methods have utilized an ink based on a water-soluble dye, but such ink involves problems such as bleeding or feathering and low weather resistance, and a pigment-based ink is therefore being investigated in order to solve these problems.
However, the pigment-based ink is often inferior to the dye-based ink, for example in a storage stability over a prolonged period and in a stability of discharge from an ink jet head.
Also it causes scattering or reflection of light by pigment particles, so that an image formed from the pigment-based ink is liable to be inferior in a color developing property to the dye-based ink.
In such a situation, attention is being attracted to making the organic pigment particle size smaller as a method for improving the color developing property of the pigment-based ink.
A pigment having a small particle size of 100 nm or less (hereinafter referred to as fine pigment particles) is considered to provide a color developing property comparable to that of a dye because of little influence on light scattering and an increase in the specific surface area.
For reducing the particle size of an organic pigment, mechanical methods utilizing a dispersing equipment such as a sand mill, a roll mill or a ball mill are commonly utilized.
It is all such methods can do to reduce the pigment particle size to around a primary particle size (about 100 nm).
When a further smaller particle size is necessary, preparation thereof requires a significant amount of time and costs, and a product of uniform quality is difficult to obtain in a stable manner (cf. Japanese Patent Application Laid-open No. H10-110111).
On the other hand, Japanese Patent Publication No. H06-096679 and Japanese Patent Application Laid-open No. 2004-091560 propose a method for producing fine pigment particles by dissolving an organic pigment in a solvent, and mixing the solution of the organic solvent with a poor solvent for the organic pigment, thereby resulting in re-precipitation (hereinafter called a re-precipitation process).
However, in order to produce fine pigment particles of a nanometer size by the re-precipitation process, it is necessary to include a dispersant in an excessive amount in the system, so that the fine pigment particles obtained by this process have a large amount of dispersant adsorbed on their surfaces.
When the fine pigment particles, made into a small particle size by the re-precipitation process are used in a charge generation layer of an electrophotographic photosensitive member, the dispersant contained as an impurity together with the fine pigment particles reduces the charge generating efficiency of the charge generation layer, and also leads to deterioration in the electrophotographic characteristics when repeatedly used.
When such fine pigment particles are used in a pigment-based ink, since a large amount of dispersant is contained together with the fine pigment particles in the ink, a degree of freedom of formulation may decrease and discharge characteristics may deteriorate due to an increase in the viscosity.
The aforementioned prior method for producing fine pigment particles utilizes a dispersant for the purpose of dispersing a pigment, but such a dispersant developing no color becomes an impurity when contained in the ink.
It is therefore desired to provide a method for producing a liquid composition containing fine pigment particles having a small particle size, high uniformity in size and satisfactory dispersion stability, without utilizing a dispersant that may become an impurity.